Mr. Zhao was purged from the Communist Party, and placed under house arrest until his death in January for opposing the bloody Tiananmen crackdown in 1989.

Irene Ngoo, a vice president of Singapore Press Holdings, which publishes the Straits Times, says the Chinese Embassy in Singapore said Mr. Ching is assisting security authorities in an investigation into a matter "not related" to the newspaper.

"We have no cause to doubt that, throughout his stint of reporting and commenting on China, he has conducted himself with utmost professionalism," said Ms. Ngoo. "We are in close contact with his wife in Hong Kong, and are providing her with every support and assistance."

Mr. Ching previously worked for a pro-Beijing newspaper, Wen Wei Poin, in Hong Kong, but left shortly after China crushed pro-democracy student demonstrations in Beijing's Tiananmen Square in June 1989. Hundreds are thought to have died during the incident.

Earlier this month, a Chinese reporter in Hunan province Shi Tao was sentenced to 10 years in prison for providing state secrets to foreigners. A New York Times researcher has been detained in China since October for allegedly revealing state secrets.

The Committee to Protect Journalists in New York ranks China as the world's top jailer of journalists, with at least 42 reporters jailed last year.